This invention relates to a process for separating air by adiabatic pressure swing adsorption.
In the prior art adiabatic pressure swing processes for air separation, the cycle sequence usually includes a selective adsorption step during which compressed air is introduced at the adsorbent bed inlet end thereby forming a nitrogen adsorption front, nitrogen being selectively adsorbed by most adsorbents as for example, zeolitic molecular sieves. Oxygen is also coadsorbed but substantially displaced by the more strongly held nitrogen adsorbate. Oxygen effluent gas is discharged from the opposite or discharge end of the bed at about the feed air pressure and the nitrogen adsorption front moves progressively toward the discharge end. The adsorption step is terminated when the front is intermediate the inlet and discharge ends, and the bed is cocurrently depressurized with oxygen effluent being released from the discharge end and the nitrogen adsorption front moving into the previously unloaded section closer to the discharge end. The cocurrent depressurization gas may in part be discharged as oxygen product and in part returned to other adsorbent beds for a variety of purposes, e.g. purging and pressure equilization with a purged bed for partial repressurization thereof. Cocurrent depressurization is terminated before the front reaches the discharge end so that the oxygen purity of the effluent is nearly that of the gas discharged during the preceeding adsorption step as for example described more completely in Kiyonaga U.S. Pat. No. 3,176,444.
The cocurrently depressurized bed is usually further depressurized by releasing waste gas through the inlet end, i.e. countercurrently depressurized, until the bed pressure diminishes to a desired low level for purging. Then oxygen purge gas is flowed through the bed to desorb the nitrogen adsorbate and carry same out of the system. The purged and at least partly cleaned bed is then repessurized at least partly with oxygen and/or feed air and returned to the adsorption step. One such process delivering product oxygen at nearly the feed air pressure is described in Batta U.S. Pat. No. 3,564,816, and requires at least four adsorbent beds arranged in parallel flow relation. Another process delivering product oxygen at lower, slightly above atmospheric pressure is described in Batta U.S. Pat. No. 3,636,679, and requires at least three beds arranged in parallel flow relation. Still another process requiring any two adsorbent beds arranged in parallel flow relation is described in McCombs U.S. Pat. No. 3,738,087. The latter process includes an increasing pressure adsorption step of introducing feed air to the inlet end of the partially repressurized adsorption bed at pressure higher than the aforementioned intermediate pressure, selectively adsorbing nitrogen and simultaneously discharging oxygen gas, all at relative rates such that the pressure of the adsorption bed rises from the intermediate pressure during this step to higher pressure at the end of such step.
In pilot plant tests relatively high oxygen recoveries were obtained with both three bed and four bed systems. For example, in a four bed calcium zeolite A system in which the bed diameter was 4 inches and the feed air was supplied at 70.degree.F and cycled according to the teachings of the aforementioned Batta U.S. Pat. 3,564,816, at 90% O.sub.2 product purity the oxygen recovery was 45.5%. However, in commercial-scale equipment composed of calcium zeolite A beds 26 inches in diameter, the O.sub.2 recoveries were substantially less than expected, i.e. 39.4% and 42.3% at air feed temperatures of 50.degree.F and 78.degree.F, respectively. Also, in a commercial size three bed calcium zeolite A system (26 inch bed diameter) in which the feed air was supplied at temperature of 40.degree.F, the O.sub.2 recovery was less than expected. The system stabilized at a product purity of only 66% and with an oxygen recovery of only 26.7%.
An object of this invention is to provide an improved adiabatic pressure swing process for air separation which permits oxygen recoveries in commercial size equipment which are equivalent to those obtained in small pilot plant equipment.
Other objects will be apparent from the ensuing disclosure and appended claims.